The alerts provided the first details of a 3,000-patient trial halted by Sanofi-Aventis two weeks ago. The company had cited “a significant increase in cardiovascular events.”

Multaq was approved for short-term arrhythmia lasting less than six months. Since then, the F.D.A. said, 241,000 Americans have received prescriptions.

The agency said “a critical question” remained: whether the halted study would also foretell problems with those patients who took Multaq for the approved uses. Those patients should talk to their physicians, the F.D.A. said.